Burner control



y 23, 1963 D. CLARK BURNER CONTROL Filed Nov. 23, 1966 INVENTOR LORIS D.CLARK A TT'ORNEYS United States Patent Office 3,393,966 Patented July23, 1968 3,393,966 BURNER CONTROL Loris D. Clark, Dayton, Ohio,assignor, by mesne assignments, to Koehring Company, Milwaukee, Wis., acorporation of Wisconsin Filed Nov. 23, 1966, Ser. No. 596,656 4 Claims.(Cl. 431-44) ABSTRACT OF THE DISCLOSURE A failsafe solid state burnersafety control uses a bidirectional semiconductor to control theapplication of electric current to a burner. The semiconductor iscontrolled by a photocell and a silicon controlled rectifier which applya start signal to the semiconductor and which also apply an overloadcurrent to a thermal cutout switch to trip the switch and disable thecircuit if no flame is sensed by the photocell within a predeterminedshort time interval.

Brie summary of the invention This invention relates to burner safetycontrols and more particularly to a control circuit embodying solidstate control elements.

An important object of this invention is the provision of a solid stateburner safety control for controlling the application of an AC source ofpower to a burner actuator, such as an electric valve, electric motor,ignition transformer or the like.

A further object of this invention is the provision of a burner safetycontrol in which a solid state symmetrical or bi-directional conductingdevice controls the application of electric power to the burneractuator, and a cadmium sulfide cell in combination with a siliconcontrolled rectifier senses the presence of a flame and disables theoperation of the burner if a flame does not occur within a predeterminedshort time interval following initiation of burner or motor operation.

A still further object of this invention is the provision of a burnersafety control in which the usual sensitive relay parts are eliminated,and which is quiet and fail-safe in operation, and is not subject toerratic performance from vibration, air flow or position.

Another object of this invention is the provision of a solid stateburner control which either shuts off, or prevents the initiation of,burner operation in the event of component shorting or failure.

These and other objects and advantages of the present invention will beapparent from the following description, the accompanying drawing andthe appended claims.

The single figure of the drawing is a Wiring diagram of a solid stateburner safety control constructed according to this invention.

Referring to the figure of the drawing which represents a preferredembodiment of the invention, a fuel burner is indicated generally at inFIG. 1 as including a motor 12 and an ignition transformer 13. While theburner 10 may be of any type, such as oil, gas or coal stoker, which iselectrically controlled, the invention has particular applicability tothe control of the operation of compact portable space heaters such asshown and claimed in the patent of Varvel No. 3,101,193, assigned to thesame assignee as this invention. In this heater, an electric motor 12provides the air for combustion and furtherprovides for delivery of fueloil to a nozzle, while a continuously operating transformer 13 providesignition current to starting electrodes.

The power for the burner 10 is provided by an alternating current supply15 including supply leads 16 and 17 as controlled by a line switch orthermostat 18. The flow of electricity from the power supply 15 to theburner 10 is controlled by a bi-directional triode semiconducting device20 which may be gate triggered from a blocked state to a conductingstate for either polarity of the applied voltage. The device 20 ispositioned effectively in the lead 16 in current controlling relation tothe motor 12 and the transformer 13. Any suitable symmetrical orbi-directional semiconductor may be used for the device 20 which, whengated on, permits the AC current to flow through the motor andtransformer, but when gated off provides a high impedance so that nocurrent flows. Suitable devices are silicon gate-controlled AC switchesor thyristers commonly called by a generic name triac and sold byGeneral Electric Company, Semi-Conductor Products Department, Syracuse,N.Y., under the designations of types SC40D and $0468. Alternately, twosilicon controlled rectifiers connected in reverse parallel may be usedin place of the single device 20.

Means for disabling the control circuit and stopping the operation ofthe burner 10 includes a thermally actuated cutout switch 22 including aheater element 23 and normally closed switch contacts 24. The switch 22is preferably of the manual-reset circuit breaker type in which thecontacts 24 are opened by reason of excessive current through the heater23 and which remain open until manually reset. The switch contacts 24are preferably placed in one of the lines 16 or 17 leading to the burner10 and the current control device 20, and are shown in the drawing asbeing placed in the line 17. Alternately, the contacts 24 may beconnected to ground the gate 21 of the device 20 to its cathode so as tobias the device into a non-conducting state, but the arrangement shownin the drawing is preferred by reason of its greater reliability in theevent of malfunction of the device 20, since they remove power to thefull control circuit.

The circuit further includes means for providing a start signal to thegate 21 of the device 20, and, at the same time, to provide apredetermined overload current to the heater 23 of the switch 22 until aflame has been detected, indicating that the burner is operatingnormally. For this purpose, a photoconductor fiame detector 30,preferably a cadmium sulfide cell, is physically positioned to respondto the presence of the flame at the burner 10. The cell 30 has a highresistance in the order of megohms in the absence of a flame and has asubstantially de creased resistance in the order of approximately 1,000ohms in the presence of light from the flame of the burner 10.

The cell 30 is connected to control the operation of a further or secondAC control switch, preferably a silicon controlled rectifier (SCR) 32.However, a bidirectional device, such as RCA type T2893, can be used inplace of the SCR32. The cell 30 has one lead connected to the lead 16and another lead connected tothe gate 33 of the SCR32. It will furtherbe seen that the gate 33 is connected in a bridge circuit including aresistor 34 to the line 16 and a further resistor 35 to a commonjunction point 36 leading to the heater 23. The heater resistance, whichmay be in the order of approximately 250 ohms, when combined with thatof the resistor 35, form a total resistance sufliciently high comparedto the resistor 34, that the SCR32 is biased into a conductive state assoon as power is applied to the line 16 and 17. A decoupling capacitor37 shunts the gate of SCR32, the resistor 34 and the cell 30 to line 16and acts as a transient filter across lines 15 and 17 to protect theSCR32 and keep spurious line switching transients use of the controlcircuit.

The anode and cathode circuit of the SCR32 is, in turn, connected to thecommon junction 36 through a resistor 38, and although the SCR32conducts only during half.

cycles of the alternating current input, the values of the resistance ofthe heater 23 and the resistor 38 are such that a substantial overloadcurrent is immediately applied to the heater 23 of the switch 22, whichmay be in the order of approximately 150% to 250% of the rated currentof the switch 22. If no flame is sensed by the cell 30 Within apredetermined short time period of approximately to seconds, thisoverload current will cause the switch 22 to open its contacts 24,thereby shutting off power to the burner 10. Since the contacts 24 arepositioned in the line 17 leading from the heater of the switch 22 tothe power source 15, the power is similarly interrupted to the cell 30,the SCR32. and the heater 23.

If the burner 10 functions normally, a flame will be detected by thecell 30, and its resistance will be substantially reduced. Since thecell is effectively in parallel with the resistor 34, the gate 33 isshorted to the cathode and the SCR32 is biased into a non-conductivestate.

The initial signal through SCR32 is also applied to the gate 21 byconduction through a resistor 40 and a coupling capacitor 42. Since thesignal at the junction 36 is effectively an alternating current(actually, it is a halfwave rectified current with a smaller AC signalsuperimposed by reason of the bridge elements 34, and 23) a gatingsignal is applied through the capacitor 42 to the gate 21 causing thedevice 20 to conduct symmetrically, passing AC power from the source 15to the burner 10. As soon as a flame is detected, the half-waverectified signal is removed.

Once the device 20 is triggered, the voltage developed on the capacitor42 by conduction through the heater 23 and resistor triggers the deviceon succeeding cycles, and assures continued conduction of the device andcontinued operation of the burner control of the burner 10 until suchtime as the power is again interrupted.

If no flame is sensed within a 15 to 20 second period by the cell 30,the thermal cut-out switch 22 will open due to the excessive or overloadcurrent applied through the SCR32, and the opening of the contacts 24will thus disable the entire circuit until manually reset.

The circuit of this invention is fail-safe from the standpoint ofcomponent failure. If the cadmium sulfide cell 30 fails, or the leads tothe cell 30 become disconnected or open, the cutout switch 22 will beopened within a specified period due to the overload current, in themanner described above as if no flame had occurred. If the cadmiumsulfide cell 30 sees light before the heater unit is started, or if theleads to the cell 30 are shorted, then the SCR32 will be biased off andcannot fire. Under these circumstances, there is no starting signalvoltage applied to the junction 36 to fire the device 20, and the heatercannot begin to function, since the current is blocked by the device 20.Similarly, if the SCR32 should fail, no start signal would be applied tothe device 20.

The apparatus of this invention is particularly useful where reliabilityand quiet operation are desired. It is not sensitive to vibration, inthe manner of the usual cut-out relay devices, nor is it sensitive toair flow or position of mounting on the heater. It is also fail-safe,providing full safety protection for the burner.

While the form of apparatus herein described constitutes a preferredembodiment of the invention, it is to be understood that the inventionis not limited to this precise form of apparatus, and that changes maybe made therein without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:

1. A solid state burner safety control for controlling the applicationof an AC source of power to a burner actuator in response to thepresence or absence of a flame,

comprising means defining a bi-directional semiconductor deviceconnected in current controlling relation from said source to saidburner actuator and having a control gate, a thermally operated cutoutswitch having a heater and contacts, means connecting said contacts todisable the operation of said bi-directional device upon the occurrenceof excessive current through said heater, a biasing circuit for saidbi-directional device upon the occurrence of excessive current throughsaid heater, a biasing circuit for said bi-directional device and forcontrolling the operation of said cut-out switch including aphotoconductor responsive to the presence of a flame in said burner, afurther semiconductor device controlled by said photoconductor andconnected to apply a substantial overload current to said heater tocause said contacts to open if the burner does not ignite within apredetermined short time period, said photoconductor being connected tothe control gate of said further device to bias said further device offin the presence of a flame, means including said further deviceproviding a start signal to said gate of said bi-directional deviceproviding for the initiation of conduction of said bi-directional deviceconcurrent with the conduction of said further device, a couplingcapacitor, and means connecting said heater and said capacitor to saidbi-directional device gate forming a keep alive circuit for saidbi-directional device.

2. The safety control of claim 1 wherein said silicon controlledrectifier is connected to said heater substantially at the junction ofsaid heater and capacitor for applying an overload current to saidheater and said start signal to said bi-directional device.

3. The safety control of claim 1 wherein said further device is asilicon controlled rectifier, and said photoconductor is a cadmiumsulfide cell.

4. A solid state burner safety control for controlling the applicationof an AC source of power to a burner actuator in response to thepresence or absence of a flame, comprising a controllable symmetricalconducting device connected in current controlling relation to saidburner actuator and having a control gate, a thermally operated cutoutswitch having a heater and normally closed contacts, circuit meansconnecting said heater to said gate and to one side of said source,means connecting said contacts to disable the operation of said deviceby interrupting the power thereto, a biasing circuit for said device andfor controlling the operation of said cut-out switch including a cadmiumsulfide cell responsive to the presence of a flame in said burner, asilicon controlled rectifier controlled by said cadmium sulfide cell andconnected to apply a substantial overload current to said heater at saidjunction of said heater and said device control gate during conductionof said silicon controlled rectifier to cause said contacts to open ifthe burner does not ignite within a predetermined short period, saidcadmium sulfide cell being connected to the control gate of said siliconcontrolled rectifier to bias said silicon controlled rectifier ofl? inthe presence of a flame, and circuit means including said heaterproviding a signal to said gate of said device providing for thecontinued operation of said device subsequent to the sensing of a flameby said cadmium sulfide cell.

References Cited UNITED STATES PATENTS 3,149,224 9/1964 Horne et al. 2363,204,113 8/1965 Snygg 236 3,306,339 2/1967 Barton et al. l58283,335,781 8/1967 Krump ct al.

JAMES W. WESTHAVER, Primary Examiner.

